The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
During the lifetime of a turbomachine, one of the irreversible reasons for increasing clearances is misalignment of the rotor axis relative to the stator, with the clearance closing up on one side and increasing on the other.
For turbomachines there are at least two sources of misalignment that are reversible and limited to the duration of the mission: first, that related to maneuvering loads (gyroscopic torques related to variations in the attitude of the turbomachine), and secondly, associated with the ingestion of foreign bodies (such as birds). On the other hand, a slight misalignment which is reversible and reparable can occur during the lifetime of the turbomachine. It is caused by an imbalance associated with an isolated breakdown of the rotor, whether accidental or not. When this happens a thin layer of friable (abradable) material is irreversibly removed from the working surface; this leads to an irreversible increase in the clearances and thus to an equally irreversible reduction in the turbomachine's performance.
Apart from the problems caused by loss of aerodynamic performance, the misalignment of the rotor axis relative to the stator causes the release of abradable particles in the primary flow path, which can cause engine damage (in particular by causing ventilation holes in the turbine to be blocked).
Moreover, in the event of a large accidental misalignment, the rotor casing must be able to ensure that high energy moving parts do not escape. The structure of the casing is thus designed for this ultimate event.
U.S. Pat. No. 6,406,256 B1 discloses a rotating seal between the rotor and stator of an axial turbomachine. Specifically this interpretation addresses the problem of compensating for the variation in clearance between the blade tips and an outer shell in operating conditions covering a wide temperature range, such as is typically found in the turbine section of a turbomachine. The sealing device comprises a segmented outer shell arranged around the tips of the rotor blades. Each segment is held in the stator housing wall via fingers angled in the opposite direction to, and acting with, corresponding grooves in the housing wall. In the event of the temperature of the different segments increasing, the latter will expand and lengthen slightly. This lengthening will have the effect of moving the fingers away from their initial rest positions. Because they are angled, these fingers will move radially away from the rotor, which will effectively pull the segments up, thus compensating for the closing up of the segment blade tips resulting from the increase in temperature. This compensation device is interesting but lacks, however, compensation for variations in clearance caused by phenomena other than temperature variation. Indeed, the clearance between a row of rotor blades and the outer shell may also vary depending on their speed of rotation and also as a function of any misalignment due to maneuvering loads (gyroscopic torques related to variations in the attitude of the turbomachine), and the ingestion of foreign bodies (such as birds). A misalignment between the rotor and stator will modify the physical clearance between the blade tips and the shell so that the latter will come into contact with the shell on a particular sector and the clearance will increase considerably on the opposite sector.
US patent 2008/0159850 A1 discloses a rotating seal between the rotor and stator of the turbine section of an axial turbomachine. This interpretation addresses the problem of high shell temperatures and the need to cool the shell. The proposed solution consists essentially of a quick-fastening device, facilitating assembly and dismantling of the shell. More specifically, it consists of providing a segmented shell in which each segment is provided with a dovetail cross-section circumferential rib pointing radially towards the outside of the stator. This rib works in combination with a grooved section corresponding approximately in the stator housing wall. Optionally, a spring may be located in the housing wall to exert a force on the segments, this force acting substantially radially towards the centre of rotation of the rotor. This spring is intended to apply a contact force between the surfaces of the groove and the corresponding surfaces of the dovetail section rib so as to ensure a certain degree of tightness. Seals may also be arranged laterally on the rib, between the segmented surface which is opposite to the inner surface of the shell and the inner surface of the housing wall. These seals are made of solid material because of the high temperatures at which a turbine works. The advantage of this solution is that the different constituent segments of the shell can be easily replaced by a simple translational movement relative to the housing wall. Contrary to what FIG. 3 in the patent might suggest, the constituent segments of the shell are not capable of moving radially and, even less, compensating for any misalignment or variation in clearance whatsoever.
Patent FR 2636373 A1 relates to the problem of differential thermal expansions in a gas turbine and, more specifically, to compensating for the variation in clearance between the tips of the rotor blades and the associated shell. The proposed solution consists of a single closed ring-shaped jacket mounted on a rotor casing via a series of compensators using pneumatic bellows. Compressed air is fed to the compensators to exert radial forces on the shell and thereby control the radial clearance between the blade tips and the shell. This solution, although technically interesting and potentially powerful, limits itself to compensating evenly for the clearance over the entire circumference. It is therefore not capable of compensating for variations in clearance in the event of misalignment between the rotor and stator. In addition, it requires a means of controlling the air pressure, which makes its implementation relatively expensive and prone to failure.